Dewatering systems are well known in the papermaking industry. In fact, in a papermaking process there are several areas on the machinery where dewatering is a common process. For example, in the press section of the machine one or more suction pipes are connected to a source of suction and are slotted to permit passing of a fabric such as a paper maker's press felt thereacross. Applied suction through the slots and the pipes dewaters the felt and the mixture of water and air is separated in a conventional manner. Generally in this portion of the machinery, it is advisable to use a liquid ring or positive displacement type of vacuum pump.
Generally the vacuum pumps are sized for maximum demand vacuum conditions in the suction pipe when the felt or similar fabric is new. Once this vacuum set point is accomplished and the system is initiated, the vacuum pump will run at its maximum speed when the felt is new. As the felt permeability decreases with time, the vacuum level at the suction pipe automatically increases because the vacuum pump used is a constant volume unit. Various solutions have been devised to take advantage of the increased vacuum conditions such as by automatically or manually adjusting the slot width so that a greater portion of the felt is subjected to the increased vacuum conditions at any given point in time. With present concern in energy consumption, it would be an additional advantage if the energy utilized by the vacuum pump during the life of the felt is reduced. Savings in energy consumption in this manner is extremely desirable in today's paper making environment as well as any environment where energy is being used.
Naturally in dealing with the portion of the papermaking machine employing suction pipes, and where desired vacuum level is to be maintained in the pipe, it is common to use the liquid ring or positive displacement vacuum pumps for maximum effectiveness. A means of reducing energy consumption of these type of pumps is certainly desirable and would add to the overall efficiency of the papermaking machine particularly in regard to energy conservation. A natural added result would be cost savings from an energy standpoint. As stated above, prior developments in this area have generated cost savings. For example, systems have been designed which minimize horsepower requirements for vacuum pumps. These developments are disclosed in pending application Ser. No. 100,617 filed Dec. 6, 1979, now U.S. Pat. No. 4,308,077 and Ser. No. 100,814 filed Dec. 6, 1979, now U.S. Pat. No. 4,329,201.